Audio hybrids are devices which are used in telecommunications systems to convert bidirectional signals which travel over two-wire signal paths to a pair of unidirectional signals for transmission over four-wire signal paths. A common application for an audio hybrid is to function as a two-to-four wire interface between a telecommunications terminal device and transmission equipment to which it is connected. This is because terminal devices, for example telephones, facsimile machines, and computer modems, typically have two-wire conductive paths overwhich signals travel in both directions simultaneously. The associated transmission equipment, which is used to exchange signals with other terminal devices, has separate elements for processing the signals that are transmitted from and received by the terminal device. Associated with each set of unidirectional processing elements is a two-wire unidirectional signal path. Most transmission equipment thus includes two two-wire conductive paths, a four-wire conductive path, over which signals travel to and from the separate processing elements. The audio hybrid converts the two-wire bidirectional signals from the terminal device into four-wire unidirectional signals that can be applied to the telecommunications equipment.
One type of audio hybrid is the subscriber line interface circuit, or SLIC. The SLIC comprises a set of current mirrors that convert differential signals that are transmitted across the two-wire conductive path into single-ended, or ground referenced, signals for the output on the transmit path portion of the four-wire conductive path. The SLIC also simultaneously converts input signals from the receive path portion of the four-wire conductive path into differential signals for output on the two-wire conductive path. SLICs are frequently fabricated as single integrated circuit components. One such SLIC is the Bipolar Integrated Circuit SLIC Part No. MC-3419 manufactured by the Motorola Corporation. U.S. Pat. No. 4,004,109, incorporated herein by reference, discusses how a SLIC can be assembled out of a set of current mirrors.
While SLICs are useful for separating bidirectional signals into unidirectional signals, they do not work alone, Inherent impedances of the SLIC and the terminal device to which it is attached cause a portion of the signal received by the SLIC to be reflected and retransmitted as part of the signal transmitted out of the SLIC. The reflected signal, referred to as sidetone signal, can significantly distort the desired transmitted signal. For example, during a telephone conversion a reflected sidetone signal can be heard as a "singing" high-frequency signal. Sometimes the singing signal can rise to sufficient magnitude to drown out the audio signal the listener is supposed to hear.
Consequently, it is necessary to provide an impedance path, referred to as a balance network, across the separate receive/transmit signal lines connected to most SLICs. The balance network applies a portion of the signal transmitted into or received by the SLIC, to the transmit signal generated by the SLIC. This signal, called a balancing signal, cancels the reflected sidetone component of the signal transmitted by the SLIC so that final signal is free of the sidetone signal. The magnitude of the balancing signal applied to the transmit path is controlled by the impedance of the balance network.
A disadvantage of many balance networks is that their impedances are difficult to adjust. Many balance networks are constructed so that their impedances are set at the time of manufacture. This works satisfactorily when the impedance of the associated SLIC and terminal device are also known and similarly unchanging; an appropriate balancing signal will always be applied on the transmit lines extending from the SLIC. However, if a new terminal device with a different internal impedance is connect to the SLIC, the balance network may not apply a balancing signal of appropriate level to cancel out the reflected sidetone. This can cause a sidetone signal to be emitted over the transmit side of the four-wire path connected to the SLIC.
Some balance networks have been constructed that initially automatically set the appropriate impedance level. This makes it possible to attach terminal devices with different impedances to the same SLIC. Each time the terminal device is activated, the balance network automatically sets the impedance to the appropriate value so that the necessary balancing signal is applied to the transmit wires from the SLIC. Nevertheless, these networks do not adjust for changes in terminal device impedance while the SLIC is in use. This can happen, for example, when the SLIC is connected to a number of telephones and the users pick up one telephone and put down another telephone. If the telephones have different impedances, the previously-set balance network may no longer apply the appropriate balance signal. Moreover, a fixed balance signal may also not be able to compensate for changes in signal strength that can occur as a consequence of changes in amplification of the signals received by and transmitted to the terminal device. These changes can occur because the transmission equipment that processes the signals, for example, a radiotelephone base station, may automatically readjust the signal amplification because of changes in external parameters such as background noise. In either situation, the originally established balance signal may not be sufficient to prevent the development of a sidetone signal so large that it interferes with the transmitted signal to the point that the desired signal becomes undecipherable.